Donald Trump has cancelled a planned summit with North Korea next month.
The US President has written to leader Kim Jong Un only weeks before he was due to meet Mr Kim in Singapore.
He's accused North Korea of showing "tremendous anger and open hostility" in a recent statement.
President Trump also described it as a "missed opportunity" and a "truly sad moment in history".
Image: White House
North Korea this week warned of a potential "nuclear showdown", and described US Vice President Mike Pence as "ignorant" and "stupid".
The North Korean regime has objected to senior Trump administration officials - most recently Mr Pence - suggesting that the North may end up like Libya "if Kim Jong Un doesn't make a deal" and give up its nuclear weapons.
Libya's former leader Muammar Gaddafi was killed in an uprising, years after agreeing to give up the country's atomic weapons in exchange for the easing of sanctions.
The letter from President Trump was published only hours after international journalists watched the apparent demolition of North Korea's main nuclear site.